The Real Lord of the Rings

The Real Lord of the Rings

Four hundred years after they were discovered, Saturn's
breath-taking rings remain a mystery.

February
12, 2002: Galileo Galilei was accustomed to extraordinary
discoveries. Using his primitive telescope he had found new worlds
orbiting Jupiter, watched planet-sized spots crossing the Sun,
and explored craters on the Moon. But when Galileo turned his
telescope toward Saturn in 1610, even he was amazed.

The planet looked nothing like others in the solar system.
Through 17th century optics, Saturn appeared to be one bright
star closely flanked by two dimmer ones -- a blurry suggestion
of the planet's magnificent rings.

He wanted to tell everyone what he had seen, but he also
wanted to keep his work secret while he studied the puzzling
planet. So, he published his discovery in
code: smais mr milmep oet ale umibunen ugttauir as.
Unscrambled, the anagram means "I have observed the highest
planet tri-form."

Nowadays anyone with a department store telescope can get
a better view of Saturn's rings than Galileo did. Otherwise,
matters stand much as they did four hundred years ago. First-time
observers of the planet still step back from their telescopes
speechless. And scientists are still puzzled.

"After all this time we're still not sure about the origin
of Saturn's rings," says Jeff Cuzzi, a planetary scientist
at the NASA Ames Research Center. Astronomers once thought that
Saturn's rings formed when Saturn did: 4.8 billion years ago
as the Sun and planets coalesced from a swirling cloud of interstellar
gas. "But lately," Cuzzi says,
"there's a growing awareness that Saturn's rings can't be
so old."

Cuzzi
speculates that some hundreds of millions of years ago -- a time
when the earliest dinosaurs roamed our planet -- Saturn had no
bright rings. Then, he says, something unlikely happened: "A
moon-sized object from the outer solar system might have flown
nearby Saturn where tidal forces ripped it apart. Or maybe an
asteroid smashed one of Saturn's existing moons." The debris
encircled the planet and formed the rings we see today.

Left: Saturn's rings might have formed only a few hundred
million years ago when dinosaurs and their cousins roamed our
planet. Credit: Humboldt State University [more]

Saturn's ring particles range in size from microscopic dust
to barn-sized boulders. If you assembled them all in one place,
notes Cuzzi, you would have enough material to make an icy satellite
one or two hundred kilometers wide -- much like Saturn's present-day
moon Mimas.

The debris layer is extraordinarily thin, he marvels. "Saturn's
rings are 250,000 km wide, but only a few tens of meters thick.
A sheet of paper the size of San Francisco would have about the
same ratio of width to depth." Indeed, if you made a 1-meter-wide
scale model of Saturn, the rings would be 10,000 times thinner
than a razor blade.

Cuzzi says there are two reasons to believe the rings are
young:

First, they are bright and shiny like something new. It's
no joke, he assures. The wide-spanning rings sweep up space dust
(bits of debris from comets and asteroids) as Saturn orbits the
Sun. Rings much older than a few hundred million years would
be darkened by accumulated dust. "The fact that they're
bright suggests they're young," he says.

Above: Saturn's rings are very thin. Astronomers using
the Hubble Space Telescope captured this image of the rings edge-on
in 1995. Star-like objects in the ring plane are icy satellites.
[more]

Second, small moons that orbit through the outermost regions
of the ring system are gaining angular momentum at the expense
of the rings. "During the next few hundred million years,"
explains Cuzzi, "the outer half of the rings will fall toward
the planet, and the little moons -- called shepherd
satellites -- will be flung away. This is a young dynamical
system."

The first argument (shiny rings) is less certain than the
second (angular momentum), he cautions, "because we're not
sure there's enough dust at the orbit of Saturn to pollute and
blacken the rings." NASA's Cassini spacecraft will measure
the dust population when it reaches Saturn in 2004. Then, perhaps,
there will be no doubt.

Right:
Saturn's 200 km-wide moon Mimas, also known as the "Death
Star" satellite because of its distinctive impact crater,
is about as massive as Saturn's rings. [more]

Cuzzi hopes Cassini will solve other ring-mysteries, too.
"In the early '80's," he recalls, "the Voyager
spacecraft visited Saturn and took close-up pictures that revealed
many strange things in the rings, including spokes, braids
and waves.

"Some of the waves have a spiral shape, like the spiral
arms of galaxies," says Cuzzi. To an astronaut floating
among the rings, such waves would appear to be gentle swells,
a few kilometers high and hundreds of kilometers wide. They move
around the rings every few days or weeks. "We understand
these spiral waves," he added. They're triggered by gravitational
tugs from Saturn's moons -- the same ones that are sapping the
rings' angular momentum.

Other structures, like spokes
and irregular ripples, are puzzling. Some of them might be signs
of space rocks plunging through the ring system. Others might
be spawned by tiny moonlets, as yet undiscovered, plowing through
Saturn's rings. "Cassini, which will orbit Saturn for years,
should provide some answers," he says.

Left:
Voyager 2 spacecraft images of "spokes"
and other irregular features in Saturn's rings. [Quicktime
movie]

In many science fiction tales, alien visitors are amazed by
Saturn as if there were no ringed planets back in their own solar
system. According to Cuzzi, Saturn's rings might be rare indeed.
"If they are as short-lived as we think, we're lucky to
be here at just the right time to see them."

Actually, other giant planets in our solar system do have
rings, but they are very dark and millions of times less massive
than the rings of Saturn. Jupiter's rings are made of bits of
dust that fly off Jupiter's moons when they are struck by meteorites.
No one is sure what made the black rings of Neptune and Uranus,
although Cuzzi notes they could be debris from kilometer-sized
moonlets that were struck by asteroids.

In another few hundred million years, if Cuzzi is right, Saturn's
rings will sag inward and our solar system will become a little
more ordinary. Perhaps by then star-faring humans will have seen
countless ringed planets elsewhere in the Galaxy and won't care
much what happens to Saturn. On the other hand, maybe Saturn's
rings really are a Galactic wonder, and super-engineers of the
distant future will take measures to preserve them.

No one knows.

We can only be sure that Saturn's rings are lovely now. And
if they are indeed fleeting, as such ages are reckoned for stars
and planets, their short life makes them even more wonderful.
Don't miss them!

Editor's Note: This month is a good time to see Saturn.
Northern-hemisphere sky watchers can find
it high overhead after sunset; it is one of the brighter
stars in the sky. Southern-hemisphere observers should look
near the northern horizon. On Feb. 20th, Saturn will have a close
encounter in the sky with Earth's Moon. Stay tuned to Science@NASA
for details....

Web Links and more...

Why are Saturn's rings so
thin? Cuzzi explains:
In the beginning, the rings might have been "fat" --
like a bagel or doughnut -- but they would have flattened over
time because collisions between ring particles are inelastic.
(An inelastic collision is like two lumps of clay colliding;
they don't bounce.) "If there were large vertical motions
among the ring particles, those motions would be damped by collisions."
Ring particles would "settle" into their average orbital
plane, and the rings would become more like an old-fashioned
phonograph record than a bagel.

Cassini-Huygens
Mission -- (JPL)
learn more about the Cassini spacecraft, which will reach Saturn
in 2004 to explore the ringed planet and its intriguing satellite
Titan.